This invention relates to the manufacture of Group III-V compound heterostructure devices and, more particularly, to selective etching procedures useful in such manufacture.
The availability of crystal growth technologies which provide uniform layer thickness and composition over entire 3-inch GaAs and InP wafers has allowed device designers significant freedom to utilize sophisticated structures. For example, superlattices are used to realize many new types of devices including self-enhanced electro-optic devices (SEED) and resonant tunneling bipolar transistors (RTBT). A less sophisticated device, though one of more immediate commercial impact, is the FET which uses these growth technologies to form abrupt material heterojunctions to control layer thickness, and hence the threshold voltage (V.sub.th). More specifically, sacrificial "stop etch" layers of slightly differing composition are incorporated into the structure. Subjecting the structure to a selective etch enables etching to proceed to a known depth across the entire wafer, thereby improving V.sub.th uniformity and simplifying processing. For the GaAs/AlGaAs compositions some of the more commonly used wet chemical etches are NH.sub.4 OH/H.sub.2 O.sub.2 at pH=7.05 (known as PA) and KI/I.sub.2 (dilute) for GaAs, and HF/H.sub.2 O and KI/I.sub.2 (concentrated) for AlGaAs. A variety of etchants using oxidation-reduction couples with pH adjusted for reaction product solubility have been reported, but do not appear to be in general use. Reactive ion etching in a plasma containing fluorine species has also been used to remove GaAs from AlGaAs.
The PA etch has an etch rate of .sup..about. 1000 .ANG./min. For high selectivity, the pH of this etch must be carefully controlled as selectivity drops off rapidly away from pH=7.05. Because the system is unbuffered, such control is difficult. Additionally, trenching, or rapid etching near the resist edge, is commonly observed. The latter effect, when combined with the high etch rate, makes use of this etch for layers less than 500 .ANG. thick unreliable. The KI/I.sub.2 etch, while somewhat easier to control, is less selective having a selectivity ratio (rate in GaAs/rate in AlGaAs) of only 8/1 for GaAs/Al.sub.0.20 Ga.sub.0.80 As. For higher Al concentrations, (e.g. Al.sub.0.48 Ga.sub.0.52 As) the selectivity rises to about 20/1, quite useful at that concentration. However, the Ki/I.sub.2 etch also attacks gold, making it unsuitable for use in device structures where Au may be used as a mask.
In general, the wet chemical etching of a solid is viewed as a three-step process: (1) diffusion of the reactants to the solid surface, (2) reaction with the solid surface, and (3) dissolution of the reaction products. These processes are frequently complicated by the presence of surface layers such as oxides which either already exist or are formed during the course of the etching reaction, but this model has provided the basis for a variety of etches for Group III-V compound semiconductors. In the case of GaAs the active etchant is typically an oxidizing agent such as H.sub.2 O.sub.2, Cr.sub.2 O.sub.7.sup.-2, Br.sub.2, or HNO.sub.3, although a variety of other oxidizing agents have been reported. The H.sub.2 O.sub.2 -based etches have been particularly well studies. In general, solvent systems are chosen with this reactant to promote product dissolution. Solvents include (at various concentrations in water) H.sub.2 SO.sub.4, Hcl, H.sub.3 PO.sub.4, and C.sub.6 H.sub.8 O.sub.7 on the acidic side and NaOH and NH.sub.4 OH in strongly alkaline solutions. On the other hand, J. J. Kelly et al. report in Appl. Surf. Sci., Vol. 29, p. 149 (1987) the use of EDTA and NH.sub.4 OH as complexing agents at mildly basic conditions to control the H.sub.2 O.sub.2 etch rate of GaAs. Selective etching is not discussed. Rather, they investigated this etchant at pH ranging from 6 to 12. At low pH the reaction is limited by the acid, suggesting a surface controlled reaction dependent on dissolution of surface oxides by the acid. For pH&gt;12, H.sub.2 O.sub.2 is strongly dissociated and becomes less effective as an oxidizing agent. From 12&gt;pH&gt;10 the dissolution of GaAs is kinetically controlled and primarily determined by the H.sub.2 O.sub.2 concentration. Below pH=10, formation of oxide films inhibits the etching action and addition of complexing agents is required.